From my experiences as a player and scout, I have learned that elite running backs come in all sizes, shapes and styles.

Although most teams would prefer the big, bruising back with explosive speed and big-play ability in the mold of Adrian Peterson, the success of Chris Johnson, Jamaal Charles and Maurice Jones-Drew has encouraged teams to think outside of the box when searching for a franchise back.

These diminutive backs have proven capable of shouldering the bulk of the workload with their remarkable numbers. Johnson is only a season removed from topping the 2,000-yard mark, and his three straight seasons with at least 1,200 yards makes him one of the league's most productive runners during that span.

Charles and Jones-Drew have also been impressive while stringing together multiple 1,000-yard seasons, on their way to securing spots on the AFC Pro Bowl roster. Their exceptional production prompted evaluators to look at smaller runners in a different light.

When evaluating running backs, scouts are looking for dynamic ball carriers with vision, balance, body control, and burst. Although a runner's 40-yard dash time generates a lot of buzz this time of year, coaches are more concerned with seeing a runner flash the burst and acceleration to get to the second level. Holes don't stay open long in the NFL, so runners must possess the quickness to get through cracks to have any chance of mustering consistent production.

Assessing a player's physical and mental toughness is also critical to the evaluation because of the punishment that affects the position. Even though teams are more willing than ever to use multiple runners, play callers crave consistency and durability from their featured backs.

In looking at the challenges scouts have had in evaluating this year's draft class, durability and consistency top the charts. Nearly all of the runners included in the following list have battled through a series of nagging injuries that have placed them on the sidelines for multiple games in college. Consequently, their production has fluctuated, and most have failed to surpass the 1,000-yard mark in consecutive years.

Let's take a closer look at top running backs in this year's class to see the challenges that each must overcome:

He is considered the crown jewel of this year's draft class after winning the 2009 Heisman Trophy. He is a rugged, hard-nosed runner with an excellent blend of quickness and power. He shows a knack for finding open creases and is a strong finisher after contact. He might not deliver big plays on a consistent basis, but his relentless running style will produce move-the-chains runs that keep drives alive.

When looking at Ingram's weaknesses, his lack of top-end speed heads the list. He recorded 40 times in the 4.6-second range in workouts, and sometimes the deficiency shows up on tape. He doesn't flash special ability on the edges, which makes scouts wonder if he is worthy of serious consideration in the first round. Ingram must convince teams that he has the skills to make a sudden impact as a rookie. Ingram's workouts and interviews have revealed him to be a smart football player with tremendous skill, but it might not be enough to convince a team to pull the trigger in the first round when other runners with similar skills will be available in later rounds.

After finishing as the Big Ten's second-leading rusher last season, he has unexpectedly emerged as one of the top running backs. As a big, versatile playmaker with vision, quickness and burst, he flashes big-play potential. He can slither through creases between the tackles, while also showing the acceleration to get to the corner on outside runs. His ability to get to the second level is deceptive, but his elusiveness allows him to create big plays in the open field.

While his game is built on finesse, he also shows enough toughness and power to be effective. He finishes his runs with authority and has a knack for falling forward after contact. Throw in his natural receiving skills, and it's easy to see why teams are starting to get excited about what he brings to the table as a potential featured back.

That excitement, however, is tempered by the fact that he can be categorized as a "one-year wonder" based on his dramatic surge in production during his final season. Although he topped the 1,600-yard mark in 2010, he doesn't have another 1,000-yard season under his belt, which leads to questions about his ability to sustain his productivity over the long haul. While those numbers can't be ignored, it's important to note that Leshoure has scored 36 touchdowns over the past two seasons. That kind of production reveals a runner with a strong nose for the paint and suggests that he has the toughness to pick up hard-fought yardage. With teams looking for playmakers in the backfield, he will garner strong consideration early in the draft.

It's not often that a prospect can be considered the best at his position after spending the bulk of his final year as a backup. But that's the plight of Williams, and it hasn't appeared to impact his status as one of the draft's top runners. As a hard-nosed runner with vision, quickness and power, he routinely finds cracks in the middle of the defense on downhill runs. He shows explosiveness to and through the line of scrimmage, and he flashes tremendous power attacking defenders on the second level. Although he typically finishes his runs with force, he also shows impressive shake and wiggle in the open field.

The biggest challenge to Williams' success appears to be his durability. He missed four games in 2010 and never regained a prime role in the Hokies' rotation. Although injuries are bound to occur, the fact that he wasn't able to stay healthy leads scouts to wonder if his heavy workload (293 carries) as the featured back in 2009 wore him down. For teams utilizing a "running back by committee" rotation, his durability will not be a major issue. However, teams looking for a player capable of shouldering a heavy workload (20-25 attempts) on a weekly basis will need to spend more time evaluating his medical records to see if he has the frame to function as a No. 1 back.